1. Field of the Invention
The present invention relates to an apparatus for detecting knocking of an internal combustion engine such as a gasoline engine for automobiles, and more particularly to improving reliability in the detection of knocking.
2. Prior Art
Internal combustion engines such as those for automobiles have a plurality of cylinders. A mixture of gas must be burned at proper timings in the respective cylinders. Microcomputers are usually incorporated to control the ignition timings of respective igniters and the order of fuel injection through an injector into the respective cylinders so that the mixture of gas is properly burned.
Ignition timing set at too advanced crank angles can cause abnormal vibration of the engine called "knocking" with the result of damages to the cylinders. In which case, the control parameters of the cylinders must be controlled in such a direction so as to eliminate knocking (for example, ignition timing is retarded).
FIG. 5A shows a prior art knocking detecting apparatus for internal combustion engines and FIG. 5B is a waveform diagram illustrating the operation of the apparatus in FIG. 5A.
Conventionally, the respective cylinders are fired at crank angles more advanced than about B5 deg. before TDC(Top Dead Center) and the mixture of gas actually explodes at crank angles more retarded than 10-60 deg. after TDC(A10 --A60 deg.) Accordingly, knocking due to abnormal combustion of the mixture gas occurs at this explosion timing.
A knock sensor 1 takes the form of a piezoelectric element for detecting vibration and is mounted to one of the cylinders or to the respective cylinders. When knocking occurs, the output signal A of the knock sensor 1 shows a large periodic increase in amplitude as shown in FIG. 5B. A filter 21 in an interface circuit 2 passes frequency components specific to "knocking", i.e., six to eight kHz, and a gate 22 opens to output a signal A' while a mask signal M is at L level. A BGL generator 23 detects a background noise in an output signal A' from the output signal A' so as to produce a BGL(Background Noise Level) signal, which serves a reference signal to determine whether the knocking is actually occurring. The gate 22 is opened by the mask signal M supplied from a microcomputer 4 to pass the output of filter 21 to both a comparator 24 and a BGL generator 23. When the amplitude of the output signal A' exceeds the level of BGL signal, the comparator 24 determines that knocking has occurred, and outputs an H level. An integrator 25 starts to integrate an input thereto each time it is reset by a RST signal. An A/D converter 3 converts an analog signal outputted from the integrator 25 into a digital signal VR, and sends the digital signal to the microcomputer 4. The microcomputer 4 receives the digital signal each time the cylinders are fired so as to produce an angle-to-be-retarded .theta.R on the basis of the digital signal so that the firing timing is controlled in a direction so as to eliminate knocking. The microcomputer 4 outputs to the gate 22 the mask signal M that alternates between H and L levels at a predetermined period so that the interface circuit 2 efficiently receives the output signal A. For each cylinder, the mask signal goes high at a crank angle of about B75 deg. and goes low at about B5 deg. The gate 22 is closed by the H level of mask signal M. The RST signal is outputted by the microcomputer 4 to the integrator 25 at a predetermined period and rises at the same time as the RST signal.
An angle-to-be-retarded controller 45 in the microcomputer 4 adds an incremental value of angle-to-be-retarded .DELTA..theta.R to the previous angle-to-be-retarded .theta.R* so as to produce a new angle-to-be-retarded .theta.R, EQU .theta.R=.theta.R*+.DELTA..theta.R (1)
where .DELTA..theta.R is given as follows: EQU .DELTA..theta.R=VR times L
When the output signal A of knock sensor 1 is processed by the interface circuit 2, the higher the level of the output signal A the higher the accuracy of knocking control operation provided by the interface circuit 2. For highest accuracy, a "full wave" amplifier (i.e., class A amplifier) is provided between the knock sensor 1 and the interface circuit 2 so as to amplify the sensor output signal A before supplying it to the interface circuit 2.
FIG. 6 shows an emitter-grounded type class A amplifier. Biasing resistors R5 and R6 provide a base biasing voltage for the base of an amplifying transistor. Tro. A resistor R7 is an emitter-biasing resistor. The signal A is supplied to the amplifier via a coupling capacitor Co which blocks a d-c component of the signal A. The amplified output is supplied to the interface circuit 2.
The collector voltage Vc of the transistor Tro is set to about 1/2 of the power supply voltage Vcc so that the amplitude of amplified signal A varies about the collector voltage Vc between Vcc and near GND level. However, the amplified signal A is saturated beyond the maximum output swing to cause distortion of signal waveform.
It should be noted that the comparator 24 compares the BGL signal with the level of the half wave of the amplified signal A, That is, only half the amplified waveform is used to determine whether knocking is occurring, Therefore, the dynamic range of the amplifier may not be sufficient.
The level of knock sensor output A depends not only on the degree of knocking but also on the operating conditions of an engine. FIG. 8 shows the relationship between engine speed Ne and sensor output signal A. The level of signal A increases with increasing engine speed Ne, so that accurate and reliable detection of knocking is seriously affected if the characteristics of interface circuit 2 changes and errors are developed in the A/D converter 3.